This allows to display the output of duration computations
as a fraction of days, hours, minutes or seconds.

Thanks to Daniel E. Doherty for bringing up this need.

* org-table.el (org-table-duration-custom-format): New
defcustom to select output format of durations computations.
(org-table-time-seconds-to-string): Use the new variable.
(org-table-eval-formula): Allow `t' as a flag, on top of `T'.
`t' will use the custom output format defined in
`org-table-duration-custom-format.